In general “boring” refers to an operation of drilling a small diameter of bore into the ground to obtain information about structures and qualities of strata. Boring is performed to obtain information inside the ground or extract petroleum, natural gas, hot spring, subterranean water and so on.
Excavations or bores are made in various magnitudes with lengths of about several to several thousand meters and diameters from several millimeters to tens of centimeters. At present, a bore or excavation is excavated by a large scale up to a diameter of several meters in order to use the bore or excavation as a vertical shaft.
Boring methods are divided into an impact-type boring in which impact is applied to a bit to crush rocks forming a bore and a rotary boring in which a bit attached to a steel pipe together with diamond and/or hard metal is rotated while being pressed against a rock forming a bore. The deepest bore or excavation in the world reaches about 8,000 m which is bored in a petroleum field in the United States.
In such a deep bore, a number of steel pipes having a predetermined length are connected in order to extend their entire length. However, the steel pipes or rods (hereinafter will be referred to as “steel pipe”) are frequently raised in order to exchange bits, insert a steel pipe (also referred to as “casing pipe”) into the bore, or perform various examinations.
A large winding machine tower is installed to this end, and power equipments are arranged around the winding machine tower together with other facilities for excavated mud, which are used to remove excavated material and thus to prevent collapse of cavity walls. Winding machine towers seen in the petroleum field indicate that boring is being performed.
Boring is also an important operation in mines for excavating subsurface ore deposits. Bores or shafts are used in the mines as drains or vents, passages for ores or waste rock, or passages for electric cables. A preliminary boring is generally performed before excavating a shaft in geological survey or exploration of ore deposits.
Although boring for mines is not performed by large scales as in well-drilling for petroleum, many techniques have been developed so that bores can be drilled in any direction or angle or to any depth. The rotary boring can extract rock core in the form of a stem to obtain the property and grade of rock and quality of ore deposit as desirable results for exploration. In the impact-type boring, rock powder extracted from a bore during excavation can be examined to judge the property and grade of rock. Such a boring for extracting “rock core” is referred to as core boring, and the boring for extracting rock powder is referred to as sludge boring.
Boring allows various measuring instruments and experimental apparatuses to be inserted into a bore so that this bore can be used as a hole for investigation of a structure or situation inside the crust. In addition, boring can be performed for preliminary investigation in dam construction or building.
An excavator in use for such boring utilizes various hammer bits of different standards and structures according to its use and stratum (geological) conditions. Types of the hammer bits are generally divided into a direct excavation-type hammer bit and an indirect excavation-type hammer bit.
The direct excavation-type hammer bit is used when stratum has a desirable condition or where boring is not shallowly performed, in which a bit housing mounted on a hammer drill is rotated to excavate the ground without using steel pipes.
The indirect excavation-type hammer bit is used where excavation is performed while steel pipes are buried in a bore, in which a bit housing and a bit mounted on a hammer drill are driven to excavate a bore wider than the diameter of steel pipes and then is pulled out through the steel pipes.
In the indirect excavation, as the hammer drill advances to a deeper subsurface position, pressure increases by large quantities so that load of great energy obstructs the hammer bit thereby causing difficulties to excavating operation.
Although hammer bits are variously priced according to the type and standard of an article, an indirect excavation-type hammer bit is high priced ranging from about 5,000 to 30,000 dollars per piece. So, excavation operation is performed vary carefully whenever a bore is drilled with the hammer bit. If the hammer bit is damaged or fractured in operation, this causes a problem that is directly related to construction cost.
However, excavating methods in conventional excavators have problems in that the entire area of a hammer bit in an excavator is not struck under uniform pressure so that striking force of an excavator piston may be partially focused in operation to instantaneously damage the hammer bit.